The effects of agents which can perturb the cytoskeleton were studied in T lymphocytes from aging mice. Cytochalasins B and E (CB, CE) interact with the T-cell mitogen, Concanavalin A (Con A), and IL2 in activation and signal transduction of splenocytes and purified resting T cells from young (4 mo.) and senescent (24 mo.) C57BL/6 mice. After stimulating single cell suspensions with Con A, CB, CE, IL2 or various combinations of these, levels of activation were assessed by tritiated thymidine incorporation and flow cytometry. No age-related differences were observed with stimulation of splenocytes and resting T cells by very low concentrations of cytochalasins which interact with high affinity binding sites, plasma membrane proteins. The highest levels of proliferation induced by Con A were unaffected by adding optimum concentrations of CB or CE, however both cytochalasins synergized with Con A at nonoptimum concentrations resulting in 2-3 fold increases in proliferation of splenocytes and resting T cells from both young and old mice. The levels reached 40-60% of the maximum induced by Con A alone. Higher concentrations of cytochalasins cause inhibition of glucose transport by CB but not by CE. In combination with mitogenic concentrations of Con A and IL2, CB inhibited mitogenesis in all resting T cells. CE, however, had differential effects on thymidine incorporation by resting T cells from young and old mice, inhibiting the young and enhancing the old. At these higher concentrations, CE binds to low affinity binding sites, actin, in the cytosol and depolymerizes actin. These results suggest that the state of polymerization of actin in microfilaments changes with age and contributes to the decline in T lymphocyte function.